1. Field of the Invention
The present invention relates to a device for draining a web, e.g., a paper web, in a paper production machine having a press section with at least one press nip (opening) through which the web is guided and thereby drained and smoothed. However, in accordance with an exemplary embodiment of the present invention, one side of the web is more strongly smoothed than the other side.
Further, the present invention relates to a method for draining a web, e.g., a paper web, in a paper production machine, in which the web is guided through at least one press nip (opening) in which the web is drained and thereby smoothed. However, in accordance with this exemplary embodiment, one side of the web in the press nip is more strongly smoothed than the other side.
2. Discussion of the Background Information
The drainage of a paper web in a press section normally takes place with the aid of felt belts which are guided through the press openings together with the paper web. The water which is pressed out of the paper web is absorbed and removed by the felt sheets. Because the felt sheets are usually guided in loops, these felt sheets must be conditioned to maintain their water-absorbing capabilities.
With devices in use today for draining paper webs, usually one side of the web is more frequently pressed in contact with the felt than the other. Out of concern for a well-supported sheet guidance and an undesirable remoistening of the paper web, the web is guided together with only one felt belt through at least the last press opening. For example, in a known press section having three press nips, the upper web side is pressed in contact with the felt three times, while the lower web side is pressed in contact with the felt in the first press nip and against an outer surface of a smooth roll in the other two openings. As a result, a significant difference in roughness values between the upper and lower sides of the raw paper occurs after the press nip. Such a difference in the roughness leads, in particular, to differing printing characteristics for the paper and, thus, is often not desired.
It is already known how to balance differing roughness values between the two sides of a paper web that is caused by differing smoothing in the web draining device, e.g., by performing a unilateral smoothing of the paper web on a side opposite to a previous unilateral smoothing, i.e., after the press section in the end treatment of the paper. It is also known how to avoid a unilateral, greater smoothing of the paper web by arranging a same number of rolls for smoothing both sides of the paper web. Such an even arrangement of smoothing rolls is, however, not always possible or desirable. In addition, the unilateral smoothing in the opposite direction during the end treatment of the paper web is relatively costly.